1. Field of the Invention
The present invention relates to the technical field of a self-excited power conversion apparatus, and more particularly to a self-excited power conversion circuit for secondary side control output power capable of adjusting the duty cycle of the secondary power output automatically and directly without the need of feeding back to the primary side to achieve a constant output power effect, after a secondary side output voltage is detected by a secondary side control chip installed in a transformer.
2. Description of the Related Art
In general, a driver circuit is installed between a main circuit and a control circuit to drive the operation of an illumination device and convert the mains power to a power with an operating voltage and a current required for outputting a load, and an isolated circuit installed is installed between the main circuit and the load of the illumination device to provide the function of controlling the voltage and current of the output power. For example, a self-excited driver circuit with a half-bridge resonant circuit achieves the effect of converting power directly and features a quick start and an anti-flickering characteristic. On the other hand, such structure is lack of a constant power control mechanism, so that the output power will increase with the input voltage, and thus giving rise of the problem of unable to maintain a stable illumination brightness and the issue of ageing quickly.
To overcome the aforementioned problems, a light emitting diode driver circuit 1 of a single-stage high power factor as disclosed in R.O.C. Utility Model No. TW M464962 and shown in FIG. 1 comprises a power factor correction (PFC) inductor 12 and an energy storage circuit 24, wherein a trigger switch SW2 of a quasi resonant half-bridge circuit 23 is electrically conducted when an energy storage capacitor C2 in the energy storage circuit 24 is charged continuously to a breakdown voltage of a bidirectional trigger diode SIDAC, and the power factor correction inductor 12 formed a charging circuit through the trigger switch SW2, such that after the energy storage capacitor C1 stores energy, the quasi resonant half-bridge circuit 23 is charged. Now, the trigger switch SW1 is electrically conducted, and the power factor correction inductor 12 charges the energy storage capacitor C1 after the charging circuit is formed through the trigger switch SW1, and the trigger switch SW1 is cut off until the oscillation of the quasi resonant half-bridge circuit 23 changes phases, and the trigger switch SW2 is electrically conducted. By operating the trigger switches SW1, SW2 with a zero-voltage switching method, the power factor correction effect can be achieved to prevent the power rises with the sine wave of the input voltage. However, the quasi resonant half-bridge circuit 23 adopting the aforementioned method is installed in a primary side of the light emitting diode driver circuit 1, and thus resulting in an increase of the complexity of the circuit and a disadvantage of causing the overall structure of inductive impedances to have an electromagnetic interference issue easily.
In view of the aforementioned problem, it is a main subject of the present invention to maintain the simple structure of the conventional self-excited driver circuit while controlling the stability of the output power of the control circuit and providing a multi-output function and a dimming function to enhance the adaptability of being installed in various different types of illumination devices.
Therefore, the inventor of the present invention designed and developed a self-excited power conversion circuit in accordance with the present invention to overcome the aforementioned drawbacks and problems of the prior art.